The bundle of His is a small mass of electrically excitable tissue originating in the atrial septum of the heart, the wall that separates the right and left sides of the heart. The His bundle divides into the right and left bundles which connect to the Purkinje system at the apex of the heart. The Purkinje system includes electrically excitable fibers which line both ventricles. The rhythm of the normal heart originates in the thin-walled right atrium and is transmitted via the His bundle and Purkinje system, a high speed electrical conduction system, to the thick-walled ventricles which perform the pumping function of the heart.
Recordings of His bundle electrical activity have been routinely used by physicians as a diagnostic tool to, for example, localize the site of atrioventricular conduction blocks, as well as to characterize the effects of drugs on conducting tissue within the heart. However, standard surface ECG records are ineffective for detecting His signals for several reasons. Conduction through the His-Purkinje cells is much faster than through the atria or ventricular muscles. Consequently, the His signal includes significant high frequency components, typically from 100-500 Hz. Standard ECG recorders are not designed to record such high frequency signals. In addition, the bundle of His is a small mass of tissue and voltages and currents associated with His activity and are of very low magnitude. The His signal has a normal amplitude of from 0.1 to 10 microvolts on the body surface. Competing myoelectric noise, created from muscle activity in the chest and the like, is usually of equal or greater magnitude than the His signal. The surface His signal is normally indistinguishable from this background noise. Furthermore, the myoelectric noise has a frequency spectrum similar to or overlapping that of the His signal. Therefore, neither amplification nor frequency filtering of a standard surface ECG signal, nor both, will effectively distinguish the His signal from the background noise on the body surface.
The most effective way to date of accurately measuring His bundle activity has been the method suggested by Scherlag et al. in 1969, which requires cardiac catheterization. B. J. Scherlag et al., "Catheter Technique for Recording His Bundle Activity in Man," Circulation, 39:13, 1969. In this method, an electrode catheter is inserted percutaneously into the right femoral vein and advanced fluoroscopically into the right atrium so that at least two of the electrode poles straddle the tricuspid valve. The His signal appears as a high frequency spike between the P-wave and the QRS complex on the ECG tracing. While this method eliminates the problems with surface noise, it is an invasive technique which carries with it many significant risks associated with cardiac catheterization. Surface measurement of heart activity, i.e., a totally noninvasive technique, is much preferred.
Signal averaging techniques have been employed in connection with the surface measurement of His signals. See, for example, E. J. Berbari et al., "Noninvasive Technique for Detection of Electrical Activity During the P-R Segment," Circulation, 48:1005, 1973; N. C. Flowers et al., "Surface Recording of Electrical Activity from the Region of the Bundle of His," American Journal of Cardiology, 33:384, 1974; Y. Hishimoto and T. Sawayama, "Non-Invasive Recording of His Bundle Potential in Man," British Heart Journal, 37:635, 1975; R. Vincent et al., "Noninvasive Recording of Electrical Activity in the PR Segment in Man," British Heart Journal, 40:124, 1978; H. Takeda et al., "Noninvasive Recording of His-Purkinje Activity in Patients with Complete Atrioventricular Block", Circulation, 60:421, 1979; N. El-Sherif et al., "Appraisal of a Low Noise Electrocardiogram," Journal of the American College of Cardiology, 1(2):456, 1983. Signal averaging has a number of disadvantages. In order to reduce the signal to noise ratio by a factor of 10, over 100 cycles must be averaged. Thus, the system is very slow to produce a desirable output. Also, since averaging assumes that the underlying signal remains constant, beat-to-beat changes in sequential cardiac cycles cannot be detected. If the reference point selected (usually the QRS complex) does not remain constant or is measured inaccurately, then the sharp spike of the His signal will become smoothed out and distorted in amplitude, duration and morphology.
The use of a shielded room to exclude electrically and magnetically generated background noise has been suggested. N. C. Flowers et al., "Surface Recording of His-Purkinje Activity on an Every-beat Basis Without Digital Averaging," Circulation, 63:498, 1981; S. N. Erne et al., "Beat to Beat Surface Recording and Averaging of His-Purkinje Activity in Man," Journal of Electrocardiology, 16(4):355, 1983. While these systems do yield beat-to-beat His signal variations, they are undesirable systems in that complex electronics and a costly, stationary shielded room must be used. Moreover, the use of shielded rooms and/or averaging of large numbers of cycles has, at best, yielded only equivocal results.
The use of a technique known as time-sequenced adaptive filtering for removing noise from a measured signal has been investigated. E. R. Ferrara, Jr., "The Time-Sequenced Adaptive Filter," Stanford University, Ph.D. Thesis (1978), whose work is based on Widrow's algorithm. See, for example, B. Widrow et al., "Stationary and Nonstationary Learning Characteristics of the LMS Adaptive Filter," Proceedings of the IEEE, 64:1151, 1976. Unlike a fixed frequency filter, an adaptive filter adjusts its parameters during operation to optimize its performance. The adjustable parameters of an adaptive filter are called weights; they are continually updated by an iterative procedure or algorithm which requires only minimal a priori knowledge about the signal. The algorithm adjusts the weights according to predetermined criteria so that the output is an optimized estimate of the signal. These criteria are embedded in the algorithm that updates the weights. In essence, the adaptive filter learns the statistics of the signal initially and then tracks them.
Two problems associated with the use of the Ferrara algorithm in detecting surface His signals have been investigated. M. T. Juran, "Surface Recordings of His-Purkinje Activity Using Adaptive Filtering," Carnegie-Mellon University, Masters Thesis (1984). These problems concern the effects of correlated noise in the input signals to the adaptive filter and the means to automatically compute the coefficient controlling the rate of learning of the filter. In addition, the Ferrara algorithm cannot accurately detect large beat-to-beat variations in the location of the His signal.
Therefore, it is an object of the present invention to provide an improved method and apparatus for surface detection of His signals which utilizes the concepts of time-sequenced adaptive filtering. The present invention will preserve the amplitude and high frequency characteristics of the sharp spike His signal in a real time data processing apparatus. The device will filter out the background noise and locate the His signal in a very short time, as short as 4 or 5 beats, and will accurately record beat-to-beat changes.
It is a further object to accomplish all of these requirements in a system which is portable, safe and easy to use, does not require complex electronics or a specially shielded room, and can be implemented on a programmed microcomputer.